Sleep deprivation proved an aggravating factor in the death of legendary pop artist Michael Jackson.

Behind the controls of the Metro-North train that derailed in New York earlier this week was a tired driver, according to new reports that engineer William Rockefeller fell asleep at the wheel.

Could lack of sleep cause such a fatal mistake?

Biologically speaking, experts said, yes. Sleep deprivation affects the brain in multiple ways that can impair judgment, slow reaction times and increase the likelihood of drifting off during monotonous tasks.

"When you're sleep deprived, your brain reverts to a teenager -- it's all gas and no brake," said Michael Howell, a neurologist at the University of Minnesota, Twin Cities. "Suddenly the part of the brain that says, 'Let's think through this,' is not functioning well."

The purpose of sleep has long mystified scientists, said Maiken Nedergaard, a neuroscientist at the University of Rochester Medical Center in New York. In an evolutionary conundrum, lying unconscious for hours on end makes people and other animals vulnerable to predators. Yet, not sleeping for long enough can actually lead to dementia and death. Chronic sleep-deprivation can cause obesity, diabetes, heart disease and other ills.

Studies have shown that exhausted people do worse on tests of memory and have more trouble learning. Tired basketball players sink fewer free throws. Even golfers who fail to get enough shut-eye take more strokes to finish a round.

When it comes to accidents, sleep matters because failure to get enough rest hampers functioning of the brain's frontal lobes, which are responsible for executive judgment, or the ability to pay attention and make good decisions.

In overtired people, Howell said, imaging studies have shown that there is less blood flow to these areas in the front of the brain and brainwaves there move more slowly.

Sleep deprivation affects the brain in multiple ways that can impair judgment and slow reaction times.iStockPhoto

The result is a compromised ability to respond to things, along with a faulty tendency to do things you shouldn't have done. When the frontal lobes aren't working efficiently, people also have more difficulty paying attention during boring tasks, such as driving a car on a highway or operating a morning commuter train.

Early morning hours, like when the Metro-North train crashed, are some of the most vulnerable times for sleepy accidents, Howell said, especially for people whose circadian rhythms favor a later sleeping schedule and make it biologically difficult to function well after waking up with an alarm clock at 5 a.m.

Reports that Rockefeller had been driving for 20 minutes since his last stop and felt zoned out before the accident suggest that he probably fell asleep before the crash, Howell added.

Recently, scientists have begun to piece together an even more nuanced understanding of why sleep is so restorative. In a study published in Science this fall, Nedergaard and colleagues injected mice with a green dye that allowed them to track the movement of cerebrospinal fluid, the liquid that surrounds the brain.

As our brains do their work throughout the day, previous work had shown that cerebrospinal fluid collects the waste products of normal metabolism and functioning. Then, a network of tiny channels works like a dishwasher to regularly flush out the dirty fluid and send it to the liver for detoxification.

The new study found that sleeping facilitated the flushing of this toxic fluid, which was much slower to drain in sleep-deprived rodent brains. Nerve cells are very sensitive to the presence of waste, Nedergaard said. When surrounded by contaminated fluid, communication at the cellular level likely slows down.

"What we described is that this microscopic cleansing system turns on as soon as we fall asleep and washes the brain clean," Nedergaard said. "From our standpoint, when you're sleep deprived, you get a dirty brain."